1. Field of the Invention
The present invention relates to a multicolor light-emitting electroluminescent device. A multicolor light-emitting electroluminescent device according to the invention can be used in appliances such as personal computers, word processors, TV sets, facsimiles, audio sets, video devices, car navigation devices, electronic calculators, telephones, portable terminals, and industrial instruments.
2. Description of the Related Art
With needs for diversity and space saving in information devices, extensive development efforts for flat panel displays that attain lower power consumption and smaller occupied space than CRTs have been in progress. In particular, expectations for electroluminescent (EL) displays are rising because of their self-emitting feature and capability of high definition.
For EL devices, numerous studies have been made that focus on improving light emission efficiency and energy conversion efficiency. It is well known that one of the reasons for the limitation of the light emission efficiency of an EL element is the fact that more than half of light generated in the light-emitting layer is confined within the element or a transparent substrate, see, for example, Non-patent Document 1, i.e., Advanced Materials, Vol. 6, p. 491 (1994).
For extracting the light confined within the element or the transparent substrate to improve light emission efficiency, a microcavity or resonant cavity structure is well known, see, for example, Non-patent Document 2, Applied Physics Letters, vol. 64, p. 2486 (1994). An organic EL device applying this principle has been proposed, see, for example, Patent Documents 1 and 2, i.e., Japanese Unexamined Patent Application Publication No. H06-283271 and WO94/7344.
Application of the resonant cavity structure gives directivity to the photons generated in the light-emitting layer. Further, energy distribution of the photons or emission spectrum becomes sharp and peak light intensity grows up to from several to several tens of times higher. Thus, the resonant cavity structure achieves the effects of increasing the intensity of emitted light and giving a monochromatic property to the light obtained in the light-emitting layer.
In order to apply this resonant cavity EL element to a color display device, however, it is necessary to adjust the optical distance between a pair of mirrors composing the resonator for every pixel corresponding to red, blue, or green color. Thus, the manufacturing process is complicated. While it would be possible to use light-emitting layers corresponding to red, blue, and green colors for the pixels of respective colors and to vary the thicknesses of the light-emitting layers for adjusting the optical distance between the mirrors, the light emission efficiency and degradation rate, for example, vary from color to color, thus, changing driving characteristics between pixels to a large extent. Therefore, there are difficulties connected with making up a complete display device.
A method has been proposed to achieve a multicolor EL element in a simpler manufacturing process than the method that obtains a multicolor EL element using light-emitting layers corresponding to red (R), green (G), and blue (B) colors for subpixels of respective colors. That method, see Patent Document 3, i.e., Japanese Unexamined Patent Application Publication No. H03-152897, is a fluorescence conversion method that uses color conversion layers that absorb light emitted from the light-emitting layer and emit light with a different wavelength. Also disclosed is technology to combine the foregoing method with the resonant cavity EL element, see, Patent Documents 4 and 5, i.e., Japanese Patent No. 2838063 and Japanese Unexamined Patent Application Publication No. 2002-359076 (corresponding to U.S. Pat. No. 6,903,506). FIGS. 1(a) and 1(b) show an example of a resonant cavity organic EL element of the prior art. The resonant cavity organic EL element of FIG. 1(a) comprises a color conversion layer 540, a flattening layer 560, a semi-transparent reflective layer 552, a transparent electrode 522, a functional layer 530 that includes a light-emitting layer 532, and a reflective electrode 521. All of the layers are laminated on a transparent substrate 510. FIG. 1(a) shows an example of a resonant cavity organic EL element that has a functional layer 530 composed of a hole injection-transport layer 531, a light-emitting layer 532, and an electron injection-transport layer 533. A resonant cavity is formed by the semi-transparent reflective layer 552 and the reflective electrode 521, the latter also functioning as a non-transparent reflective layer 551. An effective optical path length 600 is determined by the semi-transparent reflective layer 552 and the reflective electrode 521 (551) and optimized according to the wavelength of light emitted from the light-emitting layer 532. The color conversion layer 540 is located outside the resonant cavity structure.
For composing a color EL display device with RGB subpixels employing resonant cavity structures and fluorescence conversion layers, as shown in FIG. 1(b), a blue color (B) subpixel radiates blue light emitted from the resonant cavity EL element, and a green color (G) subpixel and a red color subpixel radiate green and red light obtained by a red color conversion layer 540R and a green color conversion layer 540G respectively, in which the wavelength of the light emitted from the resonant cavity EL element is converted. As described previously, the light emitted from the resonant cavity EL element, which is a blue color output light, exhibits relatively strong directivity. On the other hand, the converted light radiating from, the color conversion layers 540R and 540G (red output light and green output light) is isotropic in radiation orientation.
Consequently, the color tone of the display device in FIG. 1(b) is strongly dependent on viewing angle. For example, white color at the direction normal to the display surface becomes yellowish at an oblique direction. Therefore, the device of FIG. 1(b) is not well suited for practical application.